Process and apparatus for treating green liquor

ABSTRACT

A method of and apparatus for treating a green liquor stream produced in a kraft pulp process to separate green liquor from dregs present in the stream by utilizing a filtration vessel that has a pressure differential between the green liquor inlet and outlet to drive the dregs from the green liquor. In addition, negative pressure is utilized in the filtration vessel to thereby drive water vapor from and consequently cool the filtered green liquor.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a method of and an apparatus fortreating green liquor formed in a kraft sulphate or soda pulp mill.

The kraft or sulfate process produces a high percentage of the chemicalpulp produced annually in the world. In the kraft process, wood chipsare cooked (digested) at an elevated temperature and pressure in “whiteliquor”, which is a water solution made up of primarily sodium sulfide(Na₂S) and sodium hydroxide (NaOH). The white liquor chemicallydissolves lignin from the wood. Spent cooking liquor and the pulp washwater are combined to form a weak black liquor which is thenconcentrated in an evaporator. The concentrated black liquor is fired ina recovery furnace or recovery boiler. Inorganic chemicals present inthe black liquor collect as a molten smelt at the bottom of the furnace.The smelt, consisting primarily of sodium carbonate (Na₂CO₃) and sodiumsulfide (Na₂S) is then removed from the recovery furnace/boiler anddissolved in water in the smelt tank to create green liquor.

An essential part of the process of producing kraft pulp is chemicalrecovery. The chemical recovery process includes the production of whiteliquor used in the pulping process. White liquor is produced by acausticizing process, whereby green liquor is reacted with calciumhydroxide, i.e. slaked lime, to regenerate the white liquor through thefollowing equilibrium reaction:

Na₂CO₃+Ca(OH)₂

2NaOH+CaCO₃

Green liquor also contains insoluble compounds, know as dregs, includingsome inorganics of all types, flecks of carbon, and unburned organics.Dregs must be removed in the green liquor filter or they continuethrough the recovery cycle and become a dead load in the system.Clarifying the green liquor to remove dregs improves white liquorclarification, mud settling, mud washing, and soda removal on the limekiln precoat filter.

It is known in the art to use a green liquor filter that uses pressuredifferential across the filter element to drive the separation of dregssolids from green liquor. Ordinarily both the high and low pressures inthe filter are above atmospheric pressure, in accordance with theprocess requirements in the downstream white liquor filter, wherepressurized operation always above atmospheric prevents heat loss byflashing of water to vapor and the subsequent loss of heat from thewhite liquor.

As indicated, green liquor is reacted with lime in a slaker and isconverted to white liquor which is subsequently carried to a digester.The slaker is operated as close to boiling as possible to produce a highgrade lime mud slurry. The lime mud slurry can be washed in water in oneor several steps before it is dewatered and fed into a lime mud kiln.Calcination of the lime mud takes place in the lime mud kiln, whichconverts the content of calcium carbonate to lime, which principallyconsists of calcium oxide (CaO). This lime can subsequently be used in alime slaker as described above.

Green liquor exiting the filter is typically hot, normally about 95° C.,and at times higher, and if not cooled sufficiently it can cause theslaker to boil over. In prior art processes the hot green liquorsolution is typically directed from the filter to a separate coolingdevice to reduce its temperature. The cooling device can be either aheat exchanger or vacuum flash cooler system. However, because of thechemical content of the green liquor solution, scaling, i.e., thedeposition of chemicals in solution as a solid on pipe and vessel walls,can be a significant problem with the shell and tube exchangers. Vacuumflash coolers do not scale as quickly but are an expensive alternate toheat exchangers In addition, while dregs cake washing is increasinglyrequired to meet stricter environmental requirements for dregsdischarged from the filter, an undesirable result is that the strengthof the green liquor within the filter is sometimes weakened.

Therefore, one objective of the invention is to have a method ofefficiently cooling green liquor before it is fed to the slaker. Anotherobjective is to have a cooling step for green liquor prior to the slakerin which scaling does not occur in the cooling apparatus. A furtherobjective is to concentrate the green liquor back to the optimalstrength that existed when it was fed to the green liquor filter priorto the dregs cake washing.

These objects and other features and advantages of the present inventionwill be appreciated and understood by those skilled in the art from thefollowing detailed description and drawing.

DESCRIPTION OF THE DRAWING

The FIGURE is a diagrammatic view of a green liquor treatment system andmethod of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides for a novel approach in which the greenliquor filtering step is combined with the green liquor cooling step ina single apparatus.

Green liquor as produced from the smelt derived from black liquor inchemical recovery of a conventional Kraft liquor cycle is directed to anagitated stabilization tank 11. The agitation provided turns thecontents of tank 11 over fairly quickly but not enough to shear anydregs that are present. The agitated green liquor is thereafter directedto green liquor filter 12 adapted for removing dregs from the greenliquor solution. A green liquor filter that can be advantageouslyutilized in the present invention is FLSmidth Dorr-Oliver Eimco Inc.'sGreen Liquor ClariDisc® Filter. Green liquor filter 12 has solutioninlet 13, solution outlet 14 and dregs outlet 15 and one or moreinternal filtration element (not shown) to filter dregs from the greenliquor solution wherein there is a pressure differential across thefiltration element, and the filter unit as a whole, from a relativelyhigher pressure at or adjacent to the filter inlet (i.e. the “upstream”side, relative to the movement of the green liquor solutiontherethrough) to a relatively lower pressure at or adjacent to thefilter outlet (i.e. the “downstream” side). Dregs removed from the greenliquor solution then exit through outlet 15 and are typically disposed.

Because of the low pressure conditions, the flashing off of vapor fromthe green liquor and the resultant cooling of the green liquor isprimarily achieved within green liquor filter unit 12 and starting onthe immediate downstream side of the unit's filtration element. As theheat of vaporization for water is approximately 1000 BTU per pound, forevery pound of water removed as vapor, approximately 1000 BTUs areremoved from the solution and the loss of one BTU will drop thetemperature of one pound of water one degree F. Since green liquor ispredominantly made up of water, it will cool with the removal of BTUsfrom the solution.

The green liquor and vapor are thereafter delivered to a green liquorseparator vessel 16. In separator vessel 16 the vapor is separated fromthe liquid. Vessel 16 optionally may also be exposed to negativepressure. Since there is no heat exchange surface per se in theseparator vessel, scaling is not a problem.

The removal of water vapor from separator 16 via outlet duct 19increases the chemical concentration in the Green Liquor. The negativepressure condition that causes the flashing is achieved with a source 17of differential pressure such as a liquid ring vacuum pump or acompressor. The vapor load created can be huge while the noncondensibleload (air and other gases that do not condense) is very small.Therefore, the liberated vapor stream, i.e., the flash vapor andnon-condensables that are present in the vapor phase off the separatorvessel are sent to condenser 18 where most of the flash vapor iscondensed either by direct contact with a cooling water stream orindirectly by a heat exchanger. But in this case, the heat exchanger isfed only vapor which is free of dissolved solids so scaling is not anissue. The method of condensation employed can determine where to directthe discharge from the condenser. For example, if a barometric condenseris employed the discharge is typically directed to weak wash or anotherprocess step demanding water. If an indirect condenser is used, thenall, none or a portion of the condensate produced can be directed backto mix with the clarified green liquor.

The concentrated and cooled green liquor is removed from separator 16via outlet duct 20. Within condenser 18 the vapor is condensed back intowater. As indicated, in one embodiment some or all of this water can bereturned to the concentrated green liquor via duct 21 depending upon thedesired strength of the green liquor solution to be delivered to theslaker, that is, if the cooled green liquor is stronger than optimum.If, because of dregs cake washing the green liquor has been dilutedbelow its optimum strength than it may be determined that none of thewater is to be combined with the concentrated green liquor. Asindicated, some or all of the condensed water can also be utilizedelsewhere in the system, such as being directed back to the filter unitto effect the dregs cake wash and is thereafter discharged from thefilter unit.

The noncondensibles (that is essentially dry gas), are preferablyrecirculated back to filter 12 to prevent oxidation of the liquor.Unlike in prior art green liquor filtration processes that use aseparate downstream flash cooling device, the non-condensibles need notbe vented to atmosphere on a continuous basis.

The pressure differential across the filtration element (and between thegreen liquor inlet and outlet of the filtration unit) in the filter canrange from about 0.2 to about 1.8 bar, and will preferably range fromabout 0.5 to about 1.5 bar. Unlike what is done in the prior art, it isimportant to maintain the low pressure end of the filter at a negativepressure to facilitate subsequent flash cooling of the green liquorsolution. By controlling how far below atmospheric the low side of thefilter unit is allowed to go, the practitioner can control how muchflashing (and subsequent cooling) occurs in the downstream flash cooler.Preferably, the negative pressure utilized on the low pressure side ofthe filter will range from just below atmospheric down to about −0.7bar. Normally, the green liquor will be cooled approximately 10° C. bythe process of the present invention. Additionally, the cleaned greenliquor will be concentrated in this process. This can be a benefit ifthe liquor is undesirably diluted within the filter due to therequirement of dregs cake washing. By controlling how much of thecondensed water is returned to the cleaned green liquor, thepractitioner can ensure the final strength of the green liquor leavingthe system is optimum.

It is to be understood that the form of this invention as shown ismerely a preferred embodiment. Various changes may be made in thefunction and arrangement of parts; equivalent means may be substitutedfor those illustrated and described; and certain features may be usedindependently from others without departing from the spirit and scope ofthe invention as defined in the following claims.

1. A method of treating a stream containing green liquor and solid dregs that is produced in a kraft pulp process, comprising (a) directing the stream to a filter unit for filtering dregs from the stream having an inlet for the stream, an outlet for filtered green liquor and an outlet for dregs, wherein the unit has at least one filtration element and a pressure differential between the filtered green liquor outlet and the inlet, with the filtered green liquor outlet being maintained at a lower pressure than the inlet, to thereby facilitate the removal of dregs from the green liquor by filtration; and (b) maintaining that portion of the filter unit downstream from the filtration element at a negative pressure to thereby drive water vapor from and cool and concentrate the filtered green liquor.
 2. The method of claim 1 comprising the further step of directing the filtered green liquor and water vapor to a separator vessel in which the vapor is separated from the green liquor.
 3. The method of claim 2 wherein the separator vessel is maintained at a negative pressure.
 4. The method of claim 2 further comprising utilizing the cooled green liquor in a recausticizing step to generate white liquor.
 5. The method of claim 1 wherein the pressure differential between the green liquor inlet and green liquor outlet ranges from about 0.2 Bar to about 1.8 Bar.
 6. The method of claim 5 wherein the pressure differential ranges from about 0.5 to about 1.5 bar.
 7. The method of claim 1 wherein the negative pressure at the green liquor outlet will range from just below atmospheric to about negative 0.7 bar.
 8. The method of claim 2 wherein the water vapor removed from the mixture is condensed back into water.
 9. The method of claim 8 wherein at least a portion of said condensed water is combined with the separated green liquor downstream from the separator vessel and prior to the recausticizing step.
 10. The method of claim 8 wherein none of said condensed water is combined with the separated green liquor.
 11. The method of claim 8 wherein at least a portion of the condensed water is directed to the filter unit and is utilized as dregs cake wash to wash dregs separated from the green liquor and is thereafter discharged from the filter unit.
 12. A method of treating a green liquor stream produced in a kraft pulp process to separate green liquor from the stream for use in a recausticizing step to generate white liquor, comprising (a) directing the stream to a filter unit having a green liquor inlet and a green liquor outlet, wherein the unit has a pressure differential between the green liquor inlet and green liquor outlet to facilitate the removal of dregs from the green liquor by filtration, with the green liquor outlet being maintained at a negative pressure to thereby drive water vapor from and cool the filtered green liquor; (b) directing the filtered green liquor and water vapor to a separator vessel in which the vapor is separated from the green liquor; and (c) condensing the water vapor removed from the green liquor back into water.
 13. The method of claim 12 wherein at least a portion of the condensed water is combined with the green liquor downstream from the separation step and prior to the recausticizing step.
 14. The method of claim 12 wherein none of the condensed water is combined with the green liquor.
 15. The method of claim 12 wherein at least a portion of the condensed water is directed to the filter unit and is utilized to wash dregs separated from the green liquor and is thereafter discharged from the filter unit.
 16. The method of claim 12 wherein the pressure differential between the green liquor inlet and green liquor outlet is about one bar.
 17. The method of claim 12 wherein the negative pressure at the green liquor outlet will range from just below atmospheric to about negative 0.7 bar
 18. The method of claim 12 wherein the separator vessel is maintained at a negative pressure.
 19. Apparatus for use in a recausticizing process for separating green liquor from dregs comprising a filtration vessel containing a green liquor inlet and a green liquor outlet; means to maintain a pressure differential between the green liquor inlet and the green liquor outlet to facilitate the removal of dregs from the green liquor by filtration; means to maintain the green liquor outlet at a negative pressure to thereby drive water vapor from and cool the filtered green liquor; means to remove the filtered dregs from the filtration vessel; and means to remove the water vapor and the filtered green liquor from the filtration vessel. 